1. Field of the Invention
The present invention relates to the area of air toxics control and particularly to the area of removing trace metals from flue gases.
2. Description of Related Art
The emission of trace metals from utility power plants is an important issue in light of the 1990 Clean Air Act Amendment (CAAA) on air toxics (Title III). Special attention has been given to mercury (Hg) in terms of its environmental release and impacts, and the Environmental Protection Agency (EPA) is closely scrutinizing sources that emit mercury. Mercury is present in flue gas in very low concentrations ( less than 1 ppm) and forms a number of volatile compounds that are difficult to remove. Specially designed and costly emissions-control systems are required to capture these trace amounts of volatile compounds effectively.
Several approaches have previously been adopted for removing mercury from gas streams. These techniques include passing the gas stream through a fixed or fluidized sorbent bed or structure or using a wet scrubbing system. The most common methods are often called xe2x80x9cfixed bedxe2x80x9d techniques. Approaches using fixed bed technologies normally pass the mercury containing gas through a bed consisting of sorbent particles or various structures such as honeycombs, screens, and fibers coated with sorbents. Common sorbents include activated carbon and noble metals such as gold and silver. In many cases where noble metals are used, only the surface layer of the sorbent structure is coated with the noble metal sorbent while the support underneath is made of ceramic or metallic materials. The sorbents in these fixed structures can be periodically regenerated by heating the structure and driving off the adsorbed mercury (see, for example, U.S. Pat. No. 5,409,522, which is incorporated herein by reference). The mercury driven off can be recovered or removed separately.
There are, however, several disadvantages of fixed bed systems. Gas streams such as those from power plant coal combustion contain significant fly ash that can plug the bed structures and, thus, the beds need to be removed frequently from operation for cleaning. Alternatively, these beds may be located downstream of a separate particulate collector (see, for example, U.S. Pat. No. 5,409,522, which is incorporated herein by reference). Particulate removal devices ensure that components of the flue gas such as fly ash are removed before the gas passes over the mercury removal device. The beds will also have to be taken off-line periodically for regeneration, thereby necessitating a second bed to remain on-line while the first one is regenerating. These beds also require significant space and are very difficult to retrofit into existing systems such as into the ductwork of powerplants without major modifications.
In one technique, a removable filter bag is coated with sorbent and placed in a baghouse downstream of a particulate control device (see, for example, U.S. Pat. No. 5,505,766, which is incorporated herein by reference).
In another technique, a porous tube of sorbent material is placed into the duct work through which the gas passes (see, for example, U.S. Pat. No. 5,948,143, which is incorporated herein by reference). Such a technique permits the tube of sorbent materials to be cleaned and the sorbent to be regenerated in place without having to stop the gas flow by heating the sorbent in situ and driving off the contaminants. However, application of heat to the porous tube while it is in the duct is not a convenient technique.
Therefore, a need remains for a cost-effective way of employing sorbents in the removal of trace contaminants from gas streams so that the sorbents can be renewed without heating and in a manner that causes minimum disruption of the gas flow.
Accordingly, the present invention involves the use of a sorbent structure that can be coated and recoated with sorbent without disrupting gas flow. Specifically, in one embodiment, the present invention provides a method for removing a vapor-phase contaminant from a contaminated gas stream in a duct, the method comprising: adsorbing fresh sorbent onto the surface of a sorbent structure positioned inside the duct; passing the contaminated gas stream over the fresh sorbent structure so that a vapor-phase contaminant is adsorbed by the fresh sorbent until saturated sorbent is produced; periodically removing the saturated sorbent from the sorbent structure and collecting the saturated sorbent outside of the duct; and repeating the adsorbing step with a new quantity of fresh sorbent. A preferred embodiment of the method of the present invention additionally comprises adsorbing the sorbent onto the sorbent structure by injecting the fresh sorbent into the contaminated gas stream prior to passing the contaminated gas stream over the sorbent structure. In an alternate embodiment of the method of the present invention, adsorbing the sorbent on to the sorbent structure is carried out prior to placing the sorbent structure inside the duct. In a particularly preferred embodiment of the method of the present invention, the vapor-phase contaminant comprises mercury and/or at least one substance containing mercury.
The present invention additionally involves an apparatus for removing a vapor-phase contaminant from a contaminated gas stream in a duct. In one embodiment, the apparatus comprises at least one sorbent structure; a means for adsorbing fresh sorbent onto the sorbent structure; a means for passing the contaminated gas over the sorbent structure to produce saturated sorbent; and a means for removing and collecting the saturated sorbent.
A preferred embodiment of the present invention involves injecting sorbents into the gas stream upstream of a particulate collection device and coating sorbent structures with the sorbents in such a way that the coating can be removed and reapplied without disrupting the gas flow.